This application claims foreign priority under 35 U.S.C. xc2xa7119 from Italian Patent Application Serial Number MI99 A 000744 filed Apr. 12, 1999, which is incorporated herein by reference for all purposes.
1. The Field of the Invention
The present invention provides a getter device advantageously shaped like a substrate for use in a thin film deposition system and a method for its use.
2. Background
Processes for depositing thin films onto substrates are widely used in the manufacture of a wide array of commercial products. Examples of these processes include the fabrication of integrated electronic circuits (ICs) in which circuits are formed on a semiconductor substrate; the manufacture of data storage media, such as compact disks (CDs), where a thin layer of aluminum is deposited onto a substrate of a transparent plastic; the production of computer hard disks where a magnetic material is deposited onto a substrate such as aluminum; and the production of flat panel displays in which active elements are commonly created on glass substrates. Processes for depositing thin layers are also being adapted to the developing field of micromechanical devices, where micron-scale mechanical structures are fabricated with similar techniques to those utilized in the production of ICs. The main industrial techniques for the deposition of thin layers include chemical deposition from the vapor phase and physical deposition from the vapor phase, widely known in the art as xe2x80x9cchemical vapor depositionxe2x80x9d and xe2x80x9cphysical vapor depositionxe2x80x9d respectively or by their acronyms xe2x80x9cCVDxe2x80x9d and xe2x80x9cPVDxe2x80x9d.
In CVD processing, two or more gaseous species are caused to react in an evacuated chamber containing a substrate. The reaction product forms a solid deposit on the substrate in the form of a thin film or layer. The degree to which the chamber must be evacuated will vary according to the particular CVD process employed. Some systems, those known as low-pressure or alternately ultra-high vacuum, can require initial evacuations of the deposition chamber to a pressure value in the range of 10xe2x88x928-10xe2x88x929 mbar. Hereinafter, reference to CVD processing will refer to the low-pressure variants unless stated otherwise.
Physical vapor deposition (PVD) actually encompasses a number of different techniques that all share the following common features:
a target formed of a material to be deposited, generally having the shape of a squat cylinder or a disk, is positioned in a deposition chamber in front of a substrate and parallel thereto; and
the chamber is initially evacuated to a base pressure and thereafter back-filled with an inert gas, generally argon or another noble gas, to a pressure of about 10xe2x88x922-10xe2x88x925 mbar; a potential difference of some thousands volts applied between the supports of the substrate and the target (providing the latter with a cathodic potential) generates a plasma of electrons and positive ions in the space between the substrate and the target; the positive ions are accelerated by the electric field into the target causing atoms or xe2x80x9cclustersxe2x80x9d of atoms to erode or xe2x80x9csputterxe2x80x9d off of the surface of the target and into the atmosphere of the chamber; material thus eroded condenses onto the substrate to form a thin film or layer.
As is well known in the art, commercially useful processes frequently include the deposition of a plurality of successive thin layers which may be performed in a succession of deposition chambers or in a single chamber configured to perform multiple depositions. Hybrid processes, comprising aspects of both CVD and PVD processes are also well known in the art.
It is further understood in the art that the properties of thin layer devices, particularly ICs, are strongly dependent on the presence of defects within the deposited layers. These defects are most commonly due to the inclusion of impurity atoms or molecules within the deposited layers. Consequently, it is important to minimize the possible sources of contamination in all processing steps. For example, contamination can be reduced by using components of the highest possible purity (reactive gases in the case of CVD, targets in case of PVD, and inert gases generally) and ensuring the highest cleanliness of all surfaces within the production system and especially within the gas distribution system and each deposition chamber.
Presently, to create high quality films and to do so with the greatest efficiency, thin film deposition processes are commonly performed in systems comprising a plurality of chambers, each configured for a specific operation. For example, deposition steps are performed in deposition chambers, while conditioning chambers can be configured for cleaning or thermal processing steps like pre-heating substrates. Systems comprising multiple chambers can be arranged linearly such that one is directly connected to the next. Alternatively, multiple chambers can be disposed around a central transfer chamber.
Chambers are connected to one another by means of valves that are normally opened only to allow the transfer of substrates from one chamber to another. Substrates are passed between chambers by automated substrate handling equipment, in general mechanical arms that are configured to grasp or support a substrate typically along an edge by the use of tangs, clamps, and guides. The valves and the automated handling equipment are typically configured to the dimensions of the substrates, and are thus designed to accommodate objects that are both thin and broad. Semiconductor substrates, for example, are generally circular, often with a machined flat segment or notch to indicate crystallographic orientation, with thicknesses between about 0.5 mm and about 1 mm and lateral dimensions between about 150 mm to about 300 mm. Substrates used in the production of flat panel displays, on the other hand, are commonly rectangular with thicknesses between about 1 mm and 5 mm and lateral dimensions between about 10 cm and 1 meter.
In order to guarantee the highest cleanliness possible, all chambers are generally kept under vacuum with the highest vacuum levels being maintained in the deposition chambers. As is well known in the art, higher vacuum levels are typically achieved through the use of a series of pumps, each intended to operate in a different pressure range. Evacuation is typically initiated with a low-vacuum mechanical pump (e.g. a rotary pump) that is effective down to a pressure range of about 1-10xe2x88x922 mbar. Lower pressures can be achieved with medium and high-vacuum pumps such as turbomolecular or cryogenic pumps.
A simple example of a process system comprising multiple chambers arranged around a transfer chamber will serve to illustrate the pathway traveled by a substrate. Substrates are initially arranged in a suitably shaped carrier (e.g. a cassette or a pod) that is loaded into a first chamber. The inner walls of the carrier are provided with tangs or guides for the purpose of keeping the substrates separate from each other, and to simplify to automated handling operations. A vacuum of about 10xe2x88x925-10xe2x88x926 mbar is achieved in the first chamber after the substrates are first introduced, and then a valve is opened between the first chamber and the transfer chamber. A mechanical arm removes a substrate from the carrier and transfers it to the transfer chamber where the pressure is maintained at a level lower than those in the first chamber, generally about 10xe2x88x927 mbar.
Next, a mechanical arm is employed, for example, to transfer the substrate from the transfer chamber to a deposition chamber through a second valve. The mechanical arm places the substrate on a sample holder near the center of the chamber. Typically, the sample-holder is supported on a pedestal that is moveable in some systems. The ability to heat the deposition zone, the region within the chamber surrounding the sample holder, is generally provided in deposition chambers both to help degas the pedestal during the initial stages of evacuation and to promote more homogeneous depositions. Deposition chambers are frequently provided for this purpose with heating equipment, such as electrical resistors or infra-red lamps, to heat the deposition zone either from the inside or from the outside of the chamber through one or more quartz windows.
The vacuum level required for thin film depositions in most manufacturing processes is generally about 10xe2x88x928 mbar, which requires between about 4 to about 12 hours to achieve. After a sufficient vacuum is obtained in a deposition chamber a thin layer can be deposited according to the technique for which the chamber is configured, for example CVD or PVD. The technique may also include one or more preliminary operations that need to be performed at the deposition vacuum level but before the actual deposition. After the deposition has been completed the substrate can then be transferred, again by means of automated handling equipment and transfer chambers, either to another chamber or back to a carrier to be removed from the system.
Ideally, systems for depositing thin films should always be kept isolated from the atmosphere. However, vacuum chambers must be opened periodically, for example, to perform maintenance on automated components, to clean interior surfaces, and in the case of PVD, to replace exhausted targets or to switch to a different target in order to use the chamber to deposit a different material. Each time a chamber is brought to ambient pressure its interior surfaces, the surfaces of equipment disposed within the chamber, and the surfaces of any targets will tend to adsorb atmospheric gases, in particular water vapor. These adsorbed gases are then continuously released into the chamber when the chamber is next evacuated. The balance between the release of adsorbed gases from interior surfaces, commonly known as degassing, and the gas removal (xe2x80x9cpumpingxe2x80x9d) speed of associated vacuum pumps substantially determines the base pressure of the system, where the base pressure is the lowest pressure attainable in a commercially reasonable length of time.
Lowering the base pressure necessarily results in fewer impurities in the process atmosphere and is therefore desirable. Base pressures are typically between about 10xe2x88x927 and about 10xe2x88x929 mbar. At these vacuum levels the chamber cleanliness is generally considered acceptable for starting a new cycle of depositions. It will be appreciated, however, that in some instances it is desirable to perform depositions and other operations at a preset pressure above the level of the base pressure. A preset value above the base pressure may be used, for instance, during the cleaning of a target and in other preliminary operations. Additionally, it may be desirable to use a preset value above the base pressure for depositions where film quality is acceptable at such pressures. The further value inherent any time a preset value above the base pressure is used is that it can be achieved more quickly than the base pressure. Faster pump-down cycles lead to greater throughput and therefore to greater yields per unit time.
Typically, in order to improve a base pressure, each time a chamber is evacuated after being opened to the atmosphere it is simultaneously heated to a temperature in the range of about 100xc2x0 C.-300xc2x0 C. This treatment is commonly known as xe2x80x9cbaking.xe2x80x9d During a bake the rate of degassing from the surfaces within a chamber is increased, thus removing much of the gas initially adsorbed from the atmosphere. It is well known that degassing can be further increased by increasing the pumping speed. Thus, increasing the pumping speed during baking further reduces the quantity of residual adsorbed gases on surfaces within the chamber. Put another way, more aggressive pumping during a baking operation results in a cleaner vacuum chamber.
All else being equal, providing a cleaner vacuum chamber while maintaining an equivalent pumping speed after baking will result in a lower base pressure in the chamber. This is so because lowering the amount of residual adsorbed gas on interior surfaces reduces subsequent degassing, and base pressure is a balance between degassing and pumping speed. It should also be noted that by providing a cleaner vacuum chamber while maintaining an equivalent pumping speed after baking one can more quickly achieve a specific pressure value above the base pressure. Therefore, in situations where further reducing the base pressure is not considered important, one can bring a vacuum chamber down to an operational pressure above the base pressure more quickly by improving the pumping during baking.
In the case of PVD, targets are subjected to an additional cleaning treatment after being exposed to the ambient atmosphere. This treatment, commonly known as xe2x80x9cburn-in,xe2x80x9d comprises performing a deposition on a sacrificial substrate and requires between about half an hour and about 4 hours. The deposition thus performed erodes away the contaminated surface of the target to expose a fresh clean surface. The contaminated surface material is deposited on the sacrificial, or xe2x80x9cdummy,xe2x80x9d substrate which then may be discarded.
The use of getter materials and devices inside thin film deposition chambers has already been disclosed in patent application EP-A-693626, the publications of international patent application WO 96/13620 and WO 97/17542, and in U.S. Pat. No. 5,778,682. The European patent application EP-A-926258, in the name of the assignee of the present application, also discloses the use of getter systems in PVD processes.
Getter materials that have been used for the production of prior art getter devices include metals such as zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), and vanadium (V), and the alloys of these metals. Additionally, these metals, alone or in combination, have been further combined with one or more other elements chosen from among chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), aluminum (Al), yttrium (Y), lanthanum (La), and the rare-earth elements. Commercially popular getter materials include the binary alloys Tixe2x80x94V, Zrxe2x80x94V, Zrxe2x80x94Fe and Zrxe2x80x94Ni and ternary alloys Zrxe2x80x94Mnxe2x80x94Fe and Zrxe2x80x94Vxe2x80x94Fe.
Getter materials have been formed into getter devices by various different methods according to the prior art. Getter materials may be formed through traditional metallurgical processes such as casting and working, but more commonly they are formed through powder metallurgy. Powder metallurgy allows the getter material to be prepared with a tailored particle size distribution and to be sintered into a body with good mechanical strength and a desired degree of porosity. For gettering purposes, porosity is advantageous to increase the specific surface area (surface area/gram of material) of the getter material in contact with the atmosphere to be pumped.
Where getter materials need to be applied in very thin layers other techniques are often employed. A getter material deposited on a support can be obtained, for example, by PVD. The preparation of getter devices by PVD is described in the publication of international patent application WO 97/49109. This technique provides the advantage of allowing the deposition of a getter material onto many kinds of supports, including glasses and ceramics. Advantageously, deposits obtained by PVD techniques tend not to be sources of particle contamination.
Additionally, getter materials can be deposited onto a support in the form of a powder. The deposition of powders can be carried out by cold rolling, as is well known in the field of powder metallurgy, however powders can only be applied to metallic supports. Another technique is to spray a suspension of getter particles in a suitable solvent onto a heated support, as described in patent application WO 95/23425 incorporated herein by reference. Furthermore the support may be coated with particles of a getter material according to an electrophoretic technique. In this technique, however, the support must be electrically conductive. Electrophoretic deposition techniques are described in U.S. Pat. No. 5,242,559 which is incorporated herein by reference. Finally, depositions of getter material powders onto supports can be carried out by a serygraphic technique, as described in the publication of international patent application WO 98/03987, also incorporated herein by reference. The serygraphic technique is particularly advantageous because it provides for depositing powdered getter materials onto widely different types of supports including metals and insulators. Further, the serygraphic technique allows for the formation of patterned deposits where a portion of a coated support surface, for example, remains uncoated.
The references described above that teach getter devices, however, disclose only systems that are fixed within a vacuum chamber and continue to remain in the chamber while depositions or other processes are performed. A significant disadvantage of the foregoing prior art is that substantial modifications are often required in order to configure a processing chamber to accommodate such getter devices and associated hardware such as heaters and shields. These modifications increase the size, the complexity, and the cost of a vacuum chamber, and increased size generally implies an increased internal volume and a longer evacuation time. Further, getter devices of the prior art are not easily removed from a chamber without opening it to the ambient atmosphere.
As is well known in the art, getter materials require for their operation an initial activation treatment at temperatures between about 250xc2x0 C. and about 900xc2x0 C. for times ranging between about a few minutes and about an hour. The specific time and temperature necessary to activate a getter will vary according to the particular composition of the getter material as well as other factors. It is further well known that activated getter materials cannot be exposed to partial pressures of reactive gases higher than about 10xe2x88x923 mbar, and that unactivated getter materials cannot be activated at such pressures, because either act can result in the combustion of the getter material.
It will be appreciated that not all gases are reactive with respect to a getter material, and therefore total pressures above about 10xe2x88x923 mbar can still be safe so long as the partial pressures of the reactive gases remains below about 10xe2x88x923 mbar. Reactive gases typically include species such as oxygen, water, carbon dioxide, carbon monoxide, hydrogen, and in some instances nitrogen. Examples of gases that are non-reactive include the noble gases such as helium and argon. Therefore, it would still be safe to introduce a getter material into a vacuum chamber containing argon at a pressure of about 10xe2x88x922 mbar so long as the sum of the partial pressures of any reactive gases remains below about 10xe2x88x923 mbar. It would also be safe, under such conditions, to activate a getter material.
It is an object of the present invention, therefore, to provide a getter device that can be easily transported into and out of an evacuated chamber instead of being permanently installed. It is another object of the present invention to provide a method for the use of such a getter device for increasing the pumping of a vacuum chamber to achieve either a lower base pressure or to reach a set operating pressure more quickly.
The present invention provides an easily loaded and unloaded getter device for reducing evacuation time and contamination in a vacuum chamber. The getter device comprises a getter body having essentially the shape of a substrate used in a deposition process. Forming the getter body to have the same shape as a substrate allows the getter device to be loaded and unloaded with respect to a chamber by existing automatic handling equipment already configured to load and unload substrates. Further, the shape allows the getter device to be placed into a substrate carrier for introduction into a processing system. Further still, most substrates are broad and thin, and a getter device having such a shape will advantageously have a high surface area per unit weight of getter material. Some embodiments of the present invention are shaped like semiconductor wafers use in semiconductor fabrication, while others are shaped like the rectangular substrates used in the production of flat panel displays.
A getter device of the present invention is preferably formed of a powder of at least one getter material. Fabrication from a powder provides a high specific surface area for improved gettering capacity. Additional embodiments of the present invention are directed to a getter body comprising a support and a first getter layer formed of at least one deposit of a getter material on a first face of the support. In further embodiments the getter body further comprises a second getter layer formed of at least one deposit of a getter material on a second face of the support. Still other embodiments further include a first uncoated rim portion on the first face of the support and a second uncoated rim portion on the second face of the support. The support is advantageous because it provides mechanical strength to the getter body. Uncoated rim portions allow getter devices of the present invention to be manipulated by automated handling equipment typically configured to handle substrates by their edges. Therefore, by leaving rim portions uncoated, the clamps, tangs, and guides used by automated handling equipment to transfer substrates will not come into contact with the getter layers and create particulate contamination.
Providing a first getter layer on only a first face of the support creates a getter device suitable for use in deposition systems that are configured to deposit materials on only one side of a substrate, such as in semiconductor processing and in the manufacture of compact disks (CDs). In these systems a substrate is commonly supported on a sample holder such that one face of the substrate rests in contact with the holder. Therefore, to avoid creating particulate contamination, it is desirable to leave uncoated the face of the support that would contact the holder. Similarly, some deposition systems are configured to deposit onto both sides of a substrate, such as in the manufacture of hard disks for computers. For these systems a getter device would be desirable where both sides of the support are coated with getter material to further increase its gettering capacity.
Getter devices of the present invention provide several advantages over the prior art. Getter devices of the present invention do not need to be permanently mounted within a process chamber. Consequently, getter devices of the present invention may be readily used in conjunction with any vacuum chamber configured to accept a substrate, regardless of whether the chamber already includes a getter pump. An easily transportable getter device also removes the need for mounting brackets, complex shielding to protect the getter material of the device, and additional heaters to periodically activate the getter material. Eliminating brackets, shields, and heaters allows the chamber to have a smaller internal volume so it can be more rapidly evacuated to a desired vacuum level. Removing these components can simplify the design and manufacture of vacuum chambers and therefore help reduce their cost. Further, an easily transportable getter device can be removed from a chamber without having to resort to a time consuming tear-down procedure that exposes the chamber to the ambient atmosphere and thereafter requires a lengthy re-assembly, bake-out, and pump-down.
The present invention also provides a method for increasing the yield of a manufacturing process that includes the deposition of a thin layer in a vacuum chamber. The method comprises introducing a getter device into the vacuum chamber before or during evacuation with the same automated substrate handling equipment used for transferring substrates. An unactivated getter device can be loaded before evacuation begins, and then after a pressure of about 10xe2x88x923 mbar or less has been achieved the getter material can be activated with heaters configured to heat a substrate. An activated getter device can be loaded after a pressure of about 10xe2x88x923 mbar or less has been achieved in the chamber. Some embodiments of the method include activating the getter material in a separate activation chamber before the getter device is loaded into the vacuum chamber. Further embodiments of the method provide for the use of a getter device in a vacuum chamber where the total pressure is above about 10xe2x88x923 mbar, provided that the sum of the partial pressures of all reactive gases remains below about 10xe2x88x923 mbar and the balance of the atmosphere within the chamber comprises non-reactive gases such as noble gases.
The method of the invention further includes continuing the chamber evacuation while maintaining therein the activated getter device until a desired pressure is achieved, and then removing the getter device from the chamber. Thereafter, a substrate can be loaded into the vacuum chamber and a deposition of a thin film performed. In some embodiments on the method of the present invention a preliminary operation, such as the burn-in of a PVD target, is performed in place of a deposition.
The method of the invention is advantageous because it allows the pressure within a vacuum chamber both to be reduced to a preset value more quickly and to be reduced to a lower base pressure. A lower base pressure results in lower impurity levels within the vacuum chamber leading to higher quality films and higher production yields. The method is also advantageous because it may be used with equipment that isn""t presently configured to include a getter pump.
These and other aspects and advantages of the present invention will become more apparent when the detailed description below is read in conjunction with the accompanying drawings.